Fascinating remark. I don't have perfect (abs) pitch and never heard of the relative variety. But I grew up playing piano, mostly on my own time, which means I'm extremely comfortable playing anything I hear in C, and just slightly slower at repeating things in A#. My brain has to transpose. But now I play pedal steel slide, and I have a trick to picking up any song quickly. I find the root chord of a song and intentionally forget what it was... so I'm not playing in F or A. I remap my brain at the start of that song to remember that tone as C. And then I can find everything without thinking about it. If I think "where's the 7th of C#" I'd have to do a few steps in my head. But once I've got the hands synced with an artificial idea of "C" I know exactly where my options are. So in a way it really may be better than perfect pitch, because I can remap a song's scale to a keyboard in my head and then strum one or two times between songs and "remap" the "C". I realize that makes me uh... hahah not Mozart. But it works to let me forget what I'm playing and just play.